1. Field of the Invention
The present invention relates to a transport/storage cask for a radioactive material such as spent fuel or the like.
2. Description of the Related Art
A transport/storage cask for a radioactive material such as spent fuel or the like from a nuclear power plant or the like is adapted to effectively dissipate heat generated through the decay of a radioactive material such as spent fuel or the like contained therein and to shield gamma rays and neutrons emitted from a radioactive material. Examples of such a cask are disclosed, for example, in Japanese Patent Application Laid-Open No. 7-27896 (kokai) and Japanese Patent Application Publication No. 5-39520 (kokoku).
A transport/storage cask disclosed in Japanese Patent Application Laid-Open No. 7-27896 (kokai) is composed of an inner shell made of a steel plate, an outer shell made of a steel plate, a lead layer interposed between the inner and outer shells, a neutron shield disposed on the outer surface of the outer shell, and heat radiation fins disposed on the outer surface of the neutron shield. The lead layer closely contacts the outer surface of the inner shell via a thin film of a lead-tin material so as to efficiently dissipate outward heat generated within the inner shell, such as that resulting from decay of a radioactive material. Gamma rays emitted from a radioactive material are shielded by the lad layer, and neutrons are shielded by the neutron shield. Thus, a radioactive material such as spent fuel is transported safely in the cask.
A transport/storage cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokoku) is composed of a metallic cylindrical vessel, an outer shell, a plurality of metallic heat-conductive members which are disposed adjacent to each other around the vessel and between the vessel and the outer shell, and a neutron shield material filling in each of closed spaces formed by the heat-conductive members and the outer shell. Each of the heat-conductive members has an L-shaped cross-section and is composed of a portion which extends in the longitudinal direction of the vessel so as to contact the outer surface of the vessel and a portion which extends in the radial direction of the vessel and whose end is attached to the inner surface of the outer shell.
The transport/storage cask disclosed in Japanese Patent Application Laid-Open No. 7-27896 (kokai) has an advantage that the inner shell can be made thin because the lead layer having an excellent shielding capability against gamma rays is disposed between the inner and outer shells, and an advantage that heat generated within the inner shell, such as that resulting from decay of a radioactive material, can be efficiently dissipated outward because the lead layer closely contacts the outer surface of the inner shell via the thin film of a lead-tin material. However, in order to attain a close contact between the lead layer and the outer surface of the inner shell, the lead layer is formed employing a so-called homogenizing treatment comprising the steps of: applying flux containing zinc chloride, stannous chloride, and the like to the outer surface of the inner shell; coating the outer surface with molten lead-tin material; assembling the inner and outer shells together; and casting lead between the inner and outer shells. As a result, the fabrication of the cask takes a longer period of time and involves higher costs. Further, lead must be carefully cast between the inner and outer shells so as to not introduce defects such as voids, and after casting, the cask must undergo an ultrasonic inspection for such defects. Moreover, heat generated during casting causes the inner and outer shells to deform, resulting in a nonuniform clearance between the inner and outer shells and thus forming a thinner portion in the thus-cast lead layer. It is therefore necessary to cast more lead than a required quantity corresponding to a required shielding thickness.
The transport/storage cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokoku) uses a vessel which is made of only carbon steel or made such that a lead layer is interposed between carbon steel layers, thereby shielding gamma rays. When the vessel is made of only carbon steel, the thickness thereof must be considerably large to shield gamma rays because carbon steel is inferior to lead in terms of gamma ray shielding capability. Even though the vessel is relatively thick, the heat-conductive performance thereof is relatively good, and thus no problem arises with respect to heat; however, the vessel's capacity for containing a radioactive material reduces accordingly, resulting in a reduced storage efficiency. When the vessel is made such that a lead layer is interposed between carbon steel layers, gamma ray shielding capability improves, but the heat-conductive performance deteriorates because it is difficult to interpose the lead layer between the carbon steel layers such that the lead layer contacts closely the carbon steel layers. In order to attain the close contact between the layers, the lead layer must be formed employing the homogenizing treatment, as described above in the paragraph of Japanese Patent Application Laid-Open No. 7-27896 (kokai), but this introduces the problem as described in the paragraph.